Recently, brushless DC motors have been widely used as drive sources of electric tools. Each of the brushless DC motors is, for example, a direct-current (DC) motor with no brush (brush for rectification), and uses a coil (winding) and a magnet (permanent magnet) on the stator side and on the rotator side, respectively, and sequentially supplies power driven by an inverter circuit to predetermined coils such that a rotor rotates. The inverter circuit is configured by using a high-capacity output transistor such as a field-effect transistor (FET) or an insulated gate bipolar transistor (IGBT), and is controlled according to a program executed by a micro computer. The brushless DC motors have features in which they are more superior in a torque characteristic as compared to DC motors with brushes, and thus can tighten screws, bolts, and the like against work pieces with a stronger force. Further, since the brushless DC motors use micro computers to control rotation of motors, they can implement various control patterns according to programs executed by the micro computers.
As an example of impact tools using brushless DC motors, there is known a technology of JP-A-2008-307664, for instance. In JP-A-2008-307664, a continuously rotating type striking mechanism unit is provided, and if a torque is given to a spindle through a power transmission mechanism unit (deceleration mechanism unit), a hammer engaged with the spindle to be movable in the rotation axis direction of the spindle rotates so as to rotate an anvil abutting on the hammer. Each of the hammer and the anvil has two convex hammer portions (striking portions) symmetrically disposed at two positions on a rotary plane, and the convex portions of the hammer and the convex portions of the anvil are positioned such that the convex portions of the hammer are engaged with the convex portions of the anvil in a rotation direction, and a rotary striking force is transmitted by the engagement of the convex positions. The hammer is swingable in an axial direction with respect to a spindle in a ring area surrounding the spindle. At the inner circumferential surface of the hammer, a cam groove is provided in an inverted “V” shape (almost a triangular shape). At the outer circumferential surface of the spindle, a cam groove is provided in a “V” shape in the axial direction. The hammer rotates through balls (metal balls) inserted between the cam groove of the outer circumferential surface and the cam groove of the inner circumferential surface of the hammer.